Carbamates are important chemicals, particularly since they can be converted easily into the corresponding isocyanates which are valuable and versatile starting materials, e.g. for the production of polyurethanes and related compounds.
Numerous attempts have been made over the years to find alternative routes to the well-known preparation of isocyanates (or their precursors the carbamates), especially the most important ones such as toluene diisocyanate (TDI) and methylene-4,4'-diphenyl diisocyanate (MDI) via the so-called "phosgene route". Reference is made in this respect to "Recent Advances in Isocyanate Chemistry" by S. Ozaki (Chem. Rev., 72 (1972) 457-496).
Much emphasis has been laid on the reduction of aromatic nitrocompounds with carbon monoxide and a lower alkanol in the presence of a sulphur, selenium or tellurium catalyst. A seemingly unavoidable disadvantage of such process is the presence of small but definite amounts of catalyst (especially selenium or selenium compounds) in the product so that even more research is devoted to solve the impurity problem.
Another approach is based on the use of a Group VIII metal compound or complex as catalyst together with one or more co-catalysts. Reference is made in this respect to U.S. Pat. No. 3,448,140, issued June 3, 1963, disclosing inter alia the use of a catalyst comprising iridium, rhodium, platinum, palladium, molybdenum or iron and a ligand together with a molar excess of ferric chloride as co-catalyst, and to U.S. Pat. No. 2,903,950 disclosing the use of a co-catalyst system based on iron oxide and/or iron oxide hydrate together with specific chlorides (particularly FE(II)chloride and/or FE(III)chloride complexes or hydrochlorides of tertiary amines. It is clear from the art that a promoter based on a salt of a metal which can exist in two or more valencies, notably iron, is not only essential but, moreover, also has to be used in a substantial molar excess calculated on the main catalyst. Even more complex catalytic systems have been proposed to remedy substantial corrosion problems inherent to the use of ferrous or ferric chloride.